Many current image-forming devices utilize belts made of materials such as polycarbonate, polyimide, or ethylene tetrafluoroethylene (ETFE), functioning as transport members. Transport members may transport toner, paper, or the like. Drive rollers for these systems, particularly for systems transporting toner, must exhibit tight tolerances with regard to diameter variation because changes in the outer diameter of a drive roller can result in peripheral speed changes of the roller, and, subsequently in the belt, causing misregistration of images at interfaces between the transport member and an image transfer unit. While, theoretically, diameter variances resulting in belt speed changes are compensative, there are significant noncompensative problems which arise from diameter variances across a given roller.
For example, rollers with dimensions yielding either slightly “conical” or “flared” rollers, lead to belt tracking problems which cannot be adjusted for fully by tuning the system during manufacturing. This can lead to early failure of the belt tracking and therefore the whole belt transport unit. Prior art drive rollers, which rely on grinding to achieve specific dimensions within a roller, are costly, require intensive manufacturing set-up and care, and frequent part inspection. While spray-coating allows for the development of more precisely dimensioned rollers, it has heretofore resulted in significant problems with respect to the coefficient of friction (COF) of the drive roller surface relative to the belt being driven. This is of particular concern in toner transport units wherein the drive roller surface must maintain a high relative COF without damaging the belt. Generally, the surface of a drive roller must maintain an adequate coefficient of friction over the operative life of the belt such that the belt does not slip against the drive roller, without degrading the belt.
In the past, rollers which have a rough surface defined by a grit-containing coating adhered to the surface, have been developed to provide long-lasting effective gripping between the roller and a belt. However, use of grit-coated rollers with polymeric belts is problematic because of premature wear of the belt caused by physical abrasion and indentation of the belt surface by the grit. This creates an unacceptable defect in the print.
Drive rollers with grit or bead-blasted surfaces suffer from the same drawbacks as grit-coated rollers, with the exception that they are less costly to manufacture and generally cannot achieve a reasonably high COF. This category includes rollers with some form of texture on the core surface, without any coating other than anodizing or sealing.
Rubber coatings or sleeves for drive rollers are also known and may be manufactured with effective gripping power relative to a belt without abrading the belt surface. Typically, rubber-coated drive rollers are manufactured either by molding rubber around the roller core and then grinding the outer diameter (OD) of the rubber to the necessary tolerances, or by creating rubber sleeves which are then assembled to the core and ground to the necessary dimensions. Various rubbers and other elastomers are used in molding or sleeve-assembly processes, including natural and synthetic isoprene rubbers, other high-friction rubbers such as ethylene-propylene-diene monomer (EPDM) and epichlorohydrin terpolymer rubbers.
However, many currently employed rubber-coated drive rollers do not satisfy all of the roller requisites in a cost-effective fashion. The manufacturing processes as described can be problematic in obtaining necessary diameter consistency throughout the length of the roller, and use of such rollers is labor-intensive, requiring constant inspection and adjustment in order to maintain necessary diameter variation tolerances. Also, functionally determined thermal expansion due to the thickness of the rubber is large, and requires an additional thermistor to be placed in the machine, adding to the overall cost. The thermistor must be in close proximity to the drive roller and must be able to measure the drive roller temperature within 1° Celsius to accurately control thermal expansion of the roller. Cost effective thermistors are generally unable to maintain this level of precision, leading to thermally induced belt speed changes which result in errors in plane to plane color registration of print. The capacity of rubber-coated rollers to undergo thermal expansion, then, adds both undesirable cost and registration uncertainty to the image-forming device.
In addition, rubber coated rollers can be problematic because there is typically at least some migration of curing ingredients, or other components, out of the rubber, which bleed onto the roller surface, lowering the frictional coefficient and contaminating the inside belt surface. Other components conventionally used in rubber compositions, such as processing aids, plasticizers, curatives and oils, also migrate to the inside surface of the belt. Such contaminants attract other contaminants such as dust and toner to localized spots on the belt which can cause print defects by changing the effective resistivity of the belt or physically damaging the belt itself.
Thin, polyurethane sleeves have also been developed, but suffer from being notch-sensitive and vulnerable to ripping with normal handling when the sleeve thickness is less than about 500 μm. Such sleeves are typically comprised of polyurethane with a thickness of about 0.5–1.5 mm. The sleeves stretch over the ends of the roller and provide adequate coefficients of friction. However, because they are typically relatively thick in order to accommodate handling and installation, and are comprised of polyurethane material, they exhibit unacceptable levels of thermally induced expansion.
Thus, it would be advantageous to provide an improved, cost effective drive roller which overcomes disadvantages of the prior art and, for example, maintains a high coefficient of friction (COF) to a belt over the life of the belt, does not damage the belt surface, undergoes very low thermal expansion, and/or does not exhibit chemical migration to the roller surface or the inside surface of the belt. It would also be advantageous to provide a polyurethane coating formulation which is adapted to spray coating application to drive rollers. It would be further advantageous to provide an improved drive roller that can be easily manufactured with precise dimensions and/or which exhibits higher tolerances with respect to characteristics influenced by operating conditions.